Oracle的主要文件类型有三种，分别是控制文件，数据文件和日志文件；日志文件记录了对数据库的所有操作记录，提供了一个恢复机 制，oracle将数据写入数据文件之前，需要将修改过的内容写入redo日志文件,oracle推荐最少存在两个日志文件组，每个组中存在两个或更多的 成员文件；LGWR进程根据日志组循环写，日志组的中的文件成员大小是一致的，并分配(LSN)log sequence number；当写满一个文件组的时候，Oracle会触发一个log switch切换到另一个日志组，将LSN号码加一，同时引发check point，DBWR进程将修改过的数据写入磁盘上的数据文件中；
触发lgwr的5个条件：
在一个事务提交之后
每三秒钟
当redo buffer cache三分之一满后
当redo buffer cache里面修改过的数据容量达到1M后
在dbwn进程将修改过的数据写入磁盘文件之前
SQL> select group#,thread#,sequence#,archived,status from v$log; //查看日志组总数和当前active状态的日志组，当前为1
GROUP# THREAD# SEQUENCE# ARC STATUS
---------- ---------- ---------- --- ----------------
1 1 46 NO CURRENT
2 1 44 YES INACTIVE
3 1 45 YES INACTIVE
SQL> alter system switch logfile; //手动触发一次日志切换
System altered.
SQL> select group#,thread#,sequence#,archived,status from v$log; //可以看到当前active的日志组已经切换为2
GROUP# THREAD# SEQUENCE# ARC STATUS
---------- ---------- ---------- --- ----------------
1 1 46 YES ACTIVE
2 1 47 NO CURRENT
3 1 45 YES INACTIVE
如
果LGWR进程工作很频繁，日志组很快就会被写满，这个时候就需要DBWN进程将修改过的数据写到数据文件中，否则Oracle将无法正常工
作，fast_start_mttr_target参数定义了DBWN进程在多少时间间隔内必须将redolog中的数据写入数据文件，单位为秒，这是个
影响数据库性能的关键参数；
SQL> show parameter fast_start;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
fast_start_io_target integer 0
fast_start_mttr_target integer 0
fast_start_parallel_rollback string LOW
SQL> alter system set fast_start_mttr_target=600 scope=both; //修改这个参数为600秒
System altered.
SQL> show parameter fast_start_mttr_target;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
fast_start_mttr_target integer 600
SQL> desc v$logfile; //查看日志组成员信息
Name Null? Type
----------------------------------------- -------- ----------------------------
GROUP# NUMBER
STATUS VARCHAR2(7)
TYPE VARCHAR2(7)
MEMBER VARCHAR2(513)
IS_RECOVERY_DEST_FILE VARCHAR2(3)
SQL> col member format a40;
SQL> select * from v$logfile;
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ---------------------------------------- ---
3 ONLINE /u01/oradata/yang/redo03.log NO
2 ONLINE /u01/oradata/yang/redo02.log NO
1 ONLINE /u01/oradata/yang/redo01.log NO
SQL>
alter database add logfile group 4
('/u01/oradata/yang/redo04a.rdo','/u01/oradata/yang/redo04b.rdo') size
50M; //添加一个新的日志组，组编号为4，存在两个组成员，大小各位50M
Database altered.
SQL> select * from v$logfile where group#=4;
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ---------------------------------------- ---
4 ONLINE /u01/oradata/yang/redo04a.rdo NO
4 ONLINE /u01/oradata/yang/redo04b.rdo NO
SQL> !ls -lh /u01/oradata/yang/redo04*
-rw-r----- 1 oracle oinstall 51M 09-20 09:49 /u01/oradata/yang/redo04a.rdo
-rw-r----- 1 oracle oinstall 51M 09-20 09:49 /u01/oradata/yang/redo04b.rdo
SQL> alter database add logfile member '/u01/oradata/yang/redo01.rdo' to group 1; //添加一个新的成员到日志组1
Database altered.
SQL> select * from v$logfile where group#=1; //INVALID状态时由于当前的的日志组并非active日志组
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ---------------------------------------- ---
1 ONLINE /u01/oradata/yang/redo01.log NO
1 INVALID ONLINE /u01/oradata/yang/redo01.rdo NO
SQL> select * from v$logfile order by group#;
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ---------------------------------------- ---
1 ONLINE /u01/oradata/yang/redo01.log NO
1 ONLINE /u01/oradata/yang/redo01.rdo NO
2 ONLINE /u01/oradata/yang/redo02.log NO
3 ONLINE /u01/oradata/yang/redo03.log NO
4 ONLINE /u01/oradata/yang/redo04b.rdo NO
4 ONLINE /u01/oradata/yang/redo04a.rdo NO
6 rows selected.
SQL> alter database drop logfile group 4; //删除日志组
Database altered.
SQL> select * from v$logfile order by group#;
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ---------------------------------------- ---
1 ONLINE /u01/oradata/yang/redo01.log NO
1 ONLINE /u01/oradata/yang/redo01.rdo NO
2 ONLINE /u01/oradata/yang/redo02.log NO
3 ONLINE /u01/oradata/yang/redo03.log NO
SQL> alter database drop logfile member '/u01/oradata/yang/redo01.rdo'; //删除日志组成员
Database altered.
SQL> select * from v$logfile order by group#;
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ---------------------------------------- ---
1 ONLINE /u01/oradata/yang/redo01.log NO
2 ONLINE /u01/oradata/yang/redo02.log NO
3 ONLINE /u01/oradata/yang/redo03.log NO
redo log的rename和relocation：这种操作对数据库的影响非常大，在生产环境中执行前最好要对数据库进行一次完整备份
1：可以采用添加新的组，删除旧的组来实现
2：数据库级别上实现
SQL> select * from v$logfile where group#=1;
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ---------------------------------------- ---
1 ONLINE /u01/oradata/yang/redo01.log NO
1 ONLINE /u01/oradata/yang/redo01.rdo NO
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> !
[oracle@jsb-ylw-5024 ~]$ mv /u01/oradata/yang/redo01.rdo /u01/oradata/redo01a.log
[oracle@jsb-ylw-5024 ~]$ exit
exit
SQL> startup mount
ORACLE instance started.
Total System Global Area 1620115456 bytes
Fixed Size 2213816 bytes
Variable Size 1056966728 bytes
Database Buffers 553648128 bytes
Redo Buffers 7286784 bytes
Database mounted.
SQL> alter database rename file '/u01/oradata/yang/redo01.rdo' to '/u01/oradata/redo01a.log';
Database altered.
SQL> alter database open;
Database altered.
SQL> select * from v$logfile where group#=1;
GROUP# STATUS TYPE MEMBER IS_
---------- ------- ------- ---------------------------------------- ---
1 ONLINE /u01/oradata/yang/redo01.log NO
1 ONLINE /u01/oradata/redo01a.log NO